1. Field of the Invention
The invention concerns a process for removing heavy metals and compounds containing heavy metals from the waste gas of metallurgical vessels which are charged with materials posing disposal problems and directly connected with a gas cleaning system and which are fed to a gas after-burning device. The invention further relates to a device for carrying out the process.
2. Description of the Prior Art
In high-temperature metallurgical processes, in particular smelting of residual materials containing nonferrous metal, portions of compounds containing heavy metals or the heavy metals themselves are volatilized in the prevailing temperatures of more than 1000.degree. C. In order to separate into slag which can be disposed of in dumps or landfills, or to separate reusable metal alloys and filter dust, this volatilizing reaction is deliberately aimed for in the processes for disposal of residual materials by adjusting the oxygen potential in the melt contained in the metallurgical vessels. In these processes the filter dust is separated from the process waste gases.
The problem materials containing heavy metal are contained in the dust as completely as possible and in the most concentrated form possible and can be disposed of simply and economically in such concentrated form in separate processes.
The gases containing the steam which is formed in these metallurgical processes and which contains metal or metal compounds must be cooled before being introduced into filter systems. Direct cooling of the gas by sprayed water increases the steam content in the gas and accordingly results in a lower dew point. This process becomes problematic when hygroscopic dusts (e.g., metal chlorides, metal carbonates or metal sulfates) are to be separated in the filter, because these dusts greedily absorb water and accordingly gum up the filter surfaces.
The disadvantages of this wet gas system for cloning flue gases have already been referred to in EP 0 158 689, which proposes a dry gas system in which the blast furnace gas is desulfurized by burning the hot contaminated blast furnace gas in a combustion chamber which is arranged immediately downstream of the furnace and is charged with combustion air. It was already known prior to this proposal that soiling and encrustation of the nest of tubes in the recuperators also occurs in the dry gas system. That is, when this indirect cooling is effected, greasy materials contained in the waste gas condense on the cooling surfaces where they accumulate in layers, and impede the cooling effect as the layers increase in thickness. These layers of greasy substances further cause an increased drop in pressure throughout the entire waste gas system by reducing the cross sections of the waste gas lines until, in extreme cases, gas can no longer be sucked out. EP 0 158 689 describes a process and a device in which the hot contaminated blast furnace gas is burned in combustion chambers charged with combustion air and a desulfurization agent in powder form is fed into the combustion chamber. Low-temperature air is used as a carrier for the desulfurizing powder so that the flue gas with the injected materials, i.e., the earlier air and desulfurizing agent, is cooled. This process has the disadvantage that it uses large quantities of gas, i.e., flue gas and added combustion air, which is not fed to the gas cleaning arrangement until after the after-burning. Moreover, an external gas, namely fresh air, is added to the flue gas, which can lead to uncontrollable chemical reactions of the problem materials.
Further, EP 0 299 340 B1 discloses a process for removing heavy metals and/or heavy metal compounds in which filter dust is heated in a reaction vessel at least until reaching the evaporation temperature of the heavy metals and/or heavy metal compounds to be removed and in which at least the metallic materials in the form of steam are subsequently quenched in a condenser or cooler and changed to liquid or solid state. Before these materials evaporate, very fine dust particles with particle sizes of less than 5 nm are separated out in the cold state and the residual portion of filter dust is fed to the reaction vessel. The raw dust is fed directly to the reaction vessel and the flue gas is cooled by a cooler which results in the aforementioned disadvantages of accumulating layers on the cooling surfaces.